A connecting rod (hereinafter referred to also as a conrod) 1, as shown in FIG. 9, acts to connect a piston and a crankshaft in an internal combustion engine, and to convert reciprocating up-and-down movements of the piston into the rotational driving force of the crankshaft.
In the conrod 1, a first through hole 3 is formed in one end of a long shank 2, and a second through hole 4, having a diameter larger than that of the first through hole 3, is formed in the other end. In general, the one end having the first through hole 3 is referred to as a smaller end 5, and the other end having the second through hole 4, which is wider than the smaller end 5, is referred to as a larger end 6.
This kind of conrod 1 is produced, for example, in such a manner that a single-piece rod having a rod body 7 and a cap 8 is integrally formed by forging (see FIG. 10). The larger end 6 is split into the rod body 7 and the cap 8 at notches C formed approximately at the center of the second through hole 4 (see FIG. 9), and the rod body 7 and the cap 8 are coupled by inserting bolts (not shown) into bolt holes 9.
As shown in FIGS. 11A to 11C, a major crack 100 is extended by means of a brittle fracture, whereby the split at the notcheseC proceeds. In the case that fine minor cracks 102 are generated due to branching from the major crack 100 during extending, such minor cracks 102 often grow when the conrod 1 is attached to an internal combustion engine or when the engine is driven. In some cases, the minor cracks 102 extend and become connected to each other, and as a result, a portion where the rod body 7 and the cap 8 are substantially not in contact is formed, as shown by reference numeral 104 in FIG. 11B.
The portion 104 is brittle, and thus there is a fear that when mechanical stresses are applied to the portion 104 by screwing the bolts, the portion 104 may become dropped as a fragment, as shown in FIG. 11C.
The generation of such a fragment causes problems in that the dropped fragment becomes inserted between the conrod 1 and the crankshaft, thereby reducing assembling accuracy, or the fragment may become mixed in with the engine oil, tending to deteriorate the oil rapidly. Thus, subsequent processes must tightly be managed, whereby the fragment is dropped and removed before commencing screwing of the bolts.
For example, a method is proposed in Patent Document 1 containing the steps of causing reciprocatory movement between a cap by a cylinder, thereby bringing the cap into slight contact with the cracked surface of a rod body, so as to remove particles, and then blowing off the removed particles by airflow from a nozzle.
Further, in Patent Document 2, the applicant has proposed a method of brushing the cracked surface.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-512522 (PCT Application)    Patent Document 2: Japanese Laid-Open Patent Publication No. 2001-3924